common/base/Suppliers.java

/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.base;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.VisibleForTesting;

import java.io.Serializable;
import java.util.concurrent.TimeUnit;

import javax.annotation.Nullable;

/**
 * Useful suppliers.
 *
 * <p>All methods return serializable suppliers as long as they're given
 * serializable parameters.
 *
 * @author Laurence Gonsalves
 * @author Harry Heymann
 * @since 2.0 (imported from Google Collections Library)
 */
@GwtCompatible
public final class Suppliers {
  private Suppliers() {}

  /**
   * Returns a new supplier which is the composition of the provided function
   * and supplier. In other words, the new supplier's value will be computed by
   * retrieving the value from {@code supplier}, and then applying
   * {@code function} to that value. Note that the resulting supplier will not
   * call {@code supplier} or invoke {@code function} until it is called.
   */
  public static <F, T> Supplier<T> compose(
      Function<? super F, T> function, Supplier<F> supplier) {
    Preconditions.checkNotNull(function);
    Preconditions.checkNotNull(supplier);
    return new SupplierComposition<F, T>(function, supplier);
  }

  private static class SupplierComposition<F, T>
      implements Supplier<T>, Serializable {
    final Function<? super F, T> function;
    final Supplier<F> supplier;

    SupplierComposition(Function<? super F, T> function, Supplier<F> supplier) {
      this.function = function;
      this.supplier = supplier;
    }

    @Override public T get() {
      return function.apply(supplier.get());
    }

    @Override public boolean equals(@Nullable Object obj) {
      if (obj instanceof SupplierComposition) {
        SupplierComposition<?, ?> that = (SupplierComposition<?, ?>) obj;
        return function.equals(that.function) && supplier.equals(that.supplier);
      }
      return false;
    }

    @Override public int hashCode() {
      return Objects.hashCode(function, supplier);
    }

    @Override public String toString() {
      return "Suppliers.compose(" + function + ", " + supplier + ")";
    }

    private static final long serialVersionUID = 0;
  }

  /**
   * Returns a supplier which caches the instance retrieved during the first
   * call to {@code get()} and returns that value on subsequent calls to
   * {@code get()}. See:
   * <a href="http://en.wikipedia.org/wiki/Memoization">memoization</a>
   *
   * <p>The returned supplier is thread-safe. The supplier's serialized form
   * does not contain the cached value, which will be recalculated when {@code
   * get()} is called on the reserialized instance.
   *
   * <p>If {@code delegate} is an instance created by an earlier call to {@code
   * memoize}, it is returned directly.
   */
  public static <T> Supplier<T> memoize(Supplier<T> delegate) {
    return (delegate instanceof MemoizingSupplier)
        ? delegate
        : new MemoizingSupplier<T>(Preconditions.checkNotNull(delegate));
  }

  @VisibleForTesting
  static class MemoizingSupplier<T> implements Supplier<T>, Serializable {
    final Supplier<T> delegate;
    transient volatile boolean initialized;
    // "value" does not need to be volatile; visibility piggy-backs
    // on volatile read of "initialized".
    transient T value;

    MemoizingSupplier(Supplier<T> delegate) {
      this.delegate = delegate;
    }

    @Override public T get() {
      // A 2-field variant of Double Checked Locking.
      if (!initialized) {
        synchronized (this) {
          if (!initialized) {
            T t = delegate.get();
            value = t;
            initialized = true;
            return t;
          }
        }
      }
      return value;
    }

    @Override public String toString() {
      return "Suppliers.memoize(" + delegate + ")";
    }

    private static final long serialVersionUID = 0;
  }

  /**
   * Returns a supplier that caches the instance supplied by the delegate and
   * removes the cached value after the specified time has passed. Subsequent
   * calls to {@code get()} return the cached value if the expiration time has
   * not passed. After the expiration time, a new value is retrieved, cached,
   * and returned. See:
   * <a href="http://en.wikipedia.org/wiki/Memoization">memoization</a>
   *
   * <p>The returned supplier is thread-safe. The supplier's serialized form
   * does not contain the cached value, which will be recalculated when {@code
   * get()} is called on the reserialized instance.
   *
   * @param duration the length of time after a value is created that it
   *     should stop being returned by subsequent {@code get()} calls
   * @param unit the unit that {@code duration} is expressed in
   * @throws IllegalArgumentException if {@code duration} is not positive
   * @since 2.0
   */
  public static <T> Supplier<T> memoizeWithExpiration(
      Supplier<T> delegate, long duration, TimeUnit unit) {
    return new ExpiringMemoizingSupplier<T>(delegate, duration, unit);
  }

  @VisibleForTesting static class ExpiringMemoizingSupplier<T>
      implements Supplier<T>, Serializable {
    final Supplier<T> delegate;
    final long durationNanos;
    transient volatile T value;
    // The special value 0 means "not yet initialized".
    transient volatile long expirationNanos;

    ExpiringMemoizingSupplier(
        Supplier<T> delegate, long duration, TimeUnit unit) {
      this.delegate = Preconditions.checkNotNull(delegate);
      this.durationNanos = unit.toNanos(duration);
      Preconditions.checkArgument(duration > 0);
    }

    @Override public T get() {
      // Another variant of Double Checked Locking.
      //
      // We use two volatile reads.  We could reduce this to one by
      // putting our fields into a holder class, but (at least on x86)
      // the extra memory consumption and indirection are more
      // expensive than the extra volatile reads.
      long nanos = expirationNanos;
      long now = Platform.systemNanoTime();
      if (nanos == 0 || now - nanos >= 0) {
        synchronized (this) {
          if (nanos == expirationNanos) {  // recheck for lost race
            T t = delegate.get();
            value = t;
            nanos = now + durationNanos;
            // In the very unlikely event that nanos is 0, set it to 1;
            // no one will notice 1 ns of tardiness.
            expirationNanos = (nanos == 0) ? 1 : nanos;
            return t;
          }
        }
      }
      return value;
    }

    @Override public String toString() {
      // This is a little strange if the unit the user provided was not NANOS,
      // but we don't want to store the unit just for toString
      return "Suppliers.memoizeWithExpiration(" + delegate + ", " +
          durationNanos + ", NANOS)";
    }

    private static final long serialVersionUID = 0;
  }

  /**
   * Returns a supplier that always supplies {@code instance}.
   */
  public static <T> Supplier<T> ofInstance(@Nullable T instance) {
    return new SupplierOfInstance<T>(instance);
  }

  private static class SupplierOfInstance<T>
      implements Supplier<T>, Serializable {
    final T instance;

    SupplierOfInstance(@Nullable T instance) {
      this.instance = instance;
    }

    @Override public T get() {
      return instance;
    }

    @Override public boolean equals(@Nullable Object obj) {
      if (obj instanceof SupplierOfInstance) {
        SupplierOfInstance<?> that = (SupplierOfInstance<?>) obj;
        return Objects.equal(instance, that.instance);
      }
      return false;
    }

    @Override public int hashCode() {
      return Objects.hashCode(instance);
    }

    @Override public String toString() {
      return "Suppliers.ofInstance(" + instance + ")";
    }

    private static final long serialVersionUID = 0;
  }

  /**
   * Returns a supplier whose {@code get()} method synchronizes on
   * {@code delegate} before calling it, making it thread-safe.
   */
  public static <T> Supplier<T> synchronizedSupplier(Supplier<T> delegate) {
    return new ThreadSafeSupplier<T>(Preconditions.checkNotNull(delegate));
  }

  private static class ThreadSafeSupplier<T>
      implements Supplier<T>, Serializable {
    final Supplier<T> delegate;

    ThreadSafeSupplier(Supplier<T> delegate) {
      this.delegate = delegate;
    }

    @Override public T get() {
      synchronized (delegate) {
        return delegate.get();
      }
    }

    @Override public String toString() {
      return "Suppliers.synchronizedSupplier(" + delegate + ")";
    }

    private static final long serialVersionUID = 0;
  }

  /**
   * Returns a function that accepts a supplier and returns the result of
   * invoking {@link Supplier#get} on that supplier.
   *
   * @since 8.0
   */
  @Beta
  public static <T> Function<Supplier<T>, T> supplierFunction() {
    @SuppressWarnings("unchecked") // implementation is "fully variant"
    SupplierFunction<T> sf = (SupplierFunction<T>) SupplierFunctionImpl.INSTANCE;
    return sf;
  }

  private interface SupplierFunction<T> extends Function<Supplier<T>, T> {}

  private enum SupplierFunctionImpl implements SupplierFunction<Object> {
    INSTANCE;

    // Note: This makes T a "pass-through type"
    @Override public Object apply(Supplier<Object> input) {
      return input.get();
    }

    @Override public String toString() {
      return "Suppliers.supplierFunction()";
    }
  }
}